Fittings are provided on the ends of metal tubes used as conduits in hydraulic systems and the like to facilitate the mounting of such tubes to various components of the system or to other tubes. While various methods are known for providing a fluid tight connection between a tube and a fitting, a method known as mechanical joining is widely used because of its ease and economy. Mechanical joining is the process of cold flowing metal of the tube into tight sealing engagement with that of the fitting by the use of a mechanical expanding tool.
In order for a tube to be properly mechanically joined to a fitting, the fitting must have a prepared joint which normally includes a grooved bore sized for freely admitting the end of the tube therein. Heretofore, the length of the bore has been only long as necessary to accommodate whatever number, normally three or four, of shallow annular, axially spaced locking grooves provided to prevent the separation of the tube and fitting.
The typical mechanical expanding tool includes a plurality of caged rollers which are circumferentially disposed about a tapered mandril. By inserting the tool within the end of the tube and by rotatably feeding the mandril through the rollers, such rollers are forced outwardly against the inside diameter of the tube which causes the radial expansion of the tube into tight sealing engagement with the bore of the fitting in which the tube is disposed. It will be appreciated that the tube is entirely uniform prior to being joined to the fitting.
To insure the information of a reliable joint, certain industrial standards have been developed which specify what the inside diameter of the tube should be after forming and to what depth the tube should be expanded from its end. As will be hereinafter more fully explained, such standards take into account manufacturing tolerances and safety margins which require the expansion of the tube past the end face of the prior art fittings. This expanded portion of the tube which extends past the end face has a thinner wall than that of the normal nonexpanded portion of the tube which, because of being unsupported by the fitting, makes it more susceptible to metal fatigue than any other portion of the tube. When these thinner unsupported tube portions are subjected to the higher vibrational bending stresses experienced in today's modern high pressure hydraulic systems, they frequently rupture, causing undue fluid losses. This problem is particularly severe in systems which are pressurized by piston type pumps because such pumps have a tendency to produce high speed pressure surges which frequently match one of the natural resonant frequencies of the tubes.